[APInt] Make clearUnusedBits branch free.

This makes the WordBits calculation calculate a value between 1 and 64 for the number of bits in the last word. Previously if the BitWidth was a multiple of 64 bits the WordBits value was 0 and we had to bail out early to avoid an undefined shift. Now with a value of 64 we no longer have an undefined shift issue.

This shows a 15-16k reduction in the size of the opt binary on my local x86-64 build.

llvm-svn: 301134